The present invention relates to a method and an apparatus for controlling the air-fuel ratio in an internal combustion engine.
In general, the air-fuel ratio in an internal combustion engine of a motor car in the ordinary running state is selected to be equal to or less than the stoichiometrical air-fuel ratio, in the accelerating at wide open throttle and in the hill climbing states to be equal to the value, approximately 13, corresponding to the maximum output of the engine, and in the idling state to be equal to the value chosen from the viewpoint of the stability of the engine.
In the prior art, for air-fuel ratio control in the ordinary running state, an open loop control of the carburetor is used in which some increase in specific fuel consumption occurs due to variation in manufactured engines, variation in the operation characteristic of an engine over time, and manufacturing variation in carburetors. In an electronically controlled fuel injection device, in which the volume of the intake air is measured by an intake air volume sensor and the like, the required amount of fuel is calculated by a computer device and the fuel is injected into the air intake manifold by an electromagnetic valve in accordance with the calculated required amount, a closed loop control is used in practice to maintain a stoichiometrical air-fuel ratio (approximately 15) by means of an oxygen concentration sensor in the exhaust duct. Also, closed loop control of a carburetor is used in practice for some kinds of engines, in which the amount of air-bleeding is modified of the stoichiometrical air-fuel ratio by the oxygen concentration sensor. Although these closed loop control systems can reduce the variation in the air-fuel ratio, these closed loop control systems involve a problem of incurring waste in the fuel consumption because the theoretical stoichiometric air-fuel ratio is not the air-fuel ratio for the best specific fuel consumption.
A prior art control method has been proposed in which the above described fuel consumption increase is prevented and optimum specific fuel consumption is attained. In this method, air which by-passes the carburetor is caused to dither, i.e. the air-fuel ratio is varied with a predetermined frequency to the richer and leaner side alternately, hence the direction of the air-fuel ratio which attains an improvement of the specific fuel consumption is determined, and thereby the air-fuel ratio is corrected by a subsidiary air valve which by-passes the carburetor. In this method, the engine runs for one period at an air-fuel ratio on the relatively richer side and for one period at another air-fuel ratio on the relatively leaner side, and the rotation rate N.sub.er while running under the richer side air-fuel ratio is compared with the rotation rate N.sub.el while running under the leaner side air-fuel ratio. The engine is controlled in such a manner that, if N.sub.er &gt;N.sub.el, the amount of the by-passing air is decreased, while if N.sub.er &gt;N.sub.el, the amount of the by-passing air is increased.
In the above described prior art control method, however, it is impossible to determine whether the variation of the rotation rate is caused by the variation of the air-fuel ratio or is caused by external factors such as accelerator action, ascending a slope, or descending a slope, if the change of the output of the engine is measured by the change of the rotation rate. Accordingly, there is a problem in this method that the control may be carried out in the direction opposite to the correct direction to improve the specific fuel consumption and thereby the specific fuel consumption may be deteriorated. Also, there is a difficulty in obtaining a subsidiary air valve, as used by this prior art system as the means for correcting the air-fuel ratio, which is capable of changing the flow area with high precision.
It is the main object of the present invention to provide an improved method for controlling the air-fuel ratio in an internal combustion enging in which the control is carried out in order to ensure that the specific fuel consumption in the internal combustion engine is maintained always as close to the optimum value as possible.